1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 STAT 435; 43 USC 2457).
2. Field of the Invention
This invention relates to direct current (DC) motor control systems, and more particularly to systems for controlling brushless DC motors in response to digital control signals generated by a microprocessor, computer or the like.
3. Brief Description of the Prior Art
Computers and microprocessors have found widespread use in control systems. Such control systems often utilize electric motors, e.g., of the DC brush or brushless type, to perform a variety of tasks such as positioning members, e.g., antennas, solar arrays, etc. However, conventional motors require extensive additional electronic circuitry in order to interface with the computers or microprocessors. The additional electronics may include a power module for translating feedback signals of servo modules, tachometers, shaft encoders, feedback summing boxes and potentiometers for providing appropriate shaft position, speed, etc. The use of such components to translate the digital signals from the microprocessor into the voltages and currents is necessary to drive the motors and increases the complexity, cost, size and weight of the control system.
In addition to adding to cost, etc., the control system designers often have to design the circuitry internal to many such components to provide the desired motor performance. Where the rate or speed at which the motor shaft advances from one position to another must be controlled, the designer will generally have to make a tedious selection from available shaft position sensors such as tachometers, encoders, etc., to achieve the best match since the manufacturers of the motor and shaft position sensors seldom design one component specifically for the other. Very often a compromise is the best that the designer can accomplish. These accessory components needed to interface a source (composite) of low level digital signals with electric motors also add undesired inertia and friction to the systems. The inertia and resistance of these accessory items may exceed that of the load desired to be driven.
U.S. Pat. No. 4,249,116 advocates the use of a programmable oscillator as an interface between a brushless DC motor and a computer to provide some measure of torque and speed control. However, the control system disclosed in the U.S. Pat. No. 4,249,116 is not only complex, but unsuitable for incremental position (stepping) control or for rate (speed) and torque control during transitions, i.e., between zero and the desired rate.
Incremental position control has been achieved by stepper motors and their associated digital controls. While such motors and their controls are reasonably simple and reliable, they raise other problems, such as increased power requirements, slow speed operation and low torque sensitivity. The present invention solves the above problems by providing a DC brushless electric motor and control system which is responsive to low power level digital control signals from a computer or microprocessor to cause the motor to step to a desired position or run continuously at a controlled rate and torque, reverse direction and synchronize itself with other motors.